1. Field of the Invention
The present invention relates to an apparatus and a method for detecting a decrease in a tire air pressure and a program for detecting a decrease in a tire air pressure. More particularly, the present invention relates to an apparatus and a method for detecting a decrease in a tire air pressure and a program for detecting a decrease in a tire air pressure by which a decrease in a tire air pressure is detected based on a resonant frequency of the tire of a running vehicle.
2. Description of the Related Art
Various methods or apparatuses for detecting a decrease in an air pressure of a tire attached to a vehicle have been known. Among the methods and apparatuses, an advantageous method from the viewpoint of cost has been suggested by which a decrease in an air pressure of individual tires can be detected. This method is a tire frequency method that focuses attention on a point that a tire vibration characteristic during a vehicle's running changes while depending on an internal pressure of the tire.
In the tire frequency method as described above, a resonant frequency of a rotation signal of a wheel is estimated. This resonant frequency can be accurately estimated by subjecting a time-series signal including a tire vibration component to a frequency analysis.
However, the frequency analysis requires a memory having an extremely-large capacity for performing a process such as the Fourier transform, causing a high cost of an in-vehicle computer. Thus, this method is not realistic.
To solve this, various methods have been suggested for estimating the resonant frequency with predetermined accuracy without using a process such as the Fourier transform. For example, the specification of Japanese Patent No. 3152151 discloses a method to introduce the second-order linear prediction model with regard to a time-series signal including a tire vibration component (wheel speed signal), and to identify parameters of the linear prediction model to compute a resonant frequency of the tire. This method further extracts the influence quantity of an external factor having an influence on the calculated resonant frequency to correct, in accordance with this influence quantity of the external factor, the resonant frequency or a tire air pressure estimated based on this influence quantity. The specification of Japanese Patent No. 3152151 also enumerates, as external factors having an influence on the resonant frequency, three factors, namely an outside air temperature, a vehicle velocity, and a magnitude of the vibration given to a tire from a road surface. This method suggests that the correction is performed based on any two or all of these three external factors.
Japanese Unexamined Patent Publication No. 2005-14664 discloses a method by which, when a resonant frequency as a tire air pressure evaluation value is calculated, the resonant frequency is stored while being linked with a wheel speed, the outside air temperature, and the road type at which the resonant frequency was calculated. This method estimates a decrease in a tire air pressure based on a relation between a resonant frequency calculated under a certain condition and a resonant frequency (reference value) calculated under the same condition as the above condition.
The resonant frequency of a tire in the torsional direction is generally known to be influenced by the outside air temperature. However, the vehicle velocity and the magnitude of the vibration given to a tire from a road surface have substantially little influence on the resonant frequency and this influence is sufficiently smaller than the influence by a change in the tire air pressure. The reason is that the resonant frequency of a tire in the torsional direction is a tire-specific vibration mode and is caused due to the torsional rigidity of the tire, and thus the resonant frequency does not change depending on a vehicle velocity or a manner of excitation due to a different road. FIGS. 10 to 11 illustrate an example of a frequency characteristic of a tire rotation signal calculated by the fast Fourier transform. FIG. 10 illustrates a tire frequency characteristic when a vehicle is caused to run with three types of velocities. FIG. 11 illustrates a tire frequency characteristic when a vehicle is caused to run on two types of roads. As can be seen from FIGS. 10 to 11, a change in the vehicle velocity or the road has substantially no influence on the value itself of the tire resonant frequency existing in the vicinity of 40 Hz.
However, on the other hand, the vehicle velocity and the magnitude of the vibration given to a tire from a road surface are known to have a significant influence on frequency characteristics other than a resonant frequency in the torsional direction included in a wheel rotation signal. For example, a higher vehicle velocity causes a higher vibration of a tire in the longitudinal direction, and the frequency characteristic at a further higher frequency-side than the frequency in the torsional direction changes significantly. Furthermore, higher vibration given from a road surface causes higher excitation of a tire and thus the gain of the entire frequency characteristic is increased and the vibration of the tire in the longitudinal direction at a high speed also significantly increases. Specifically, the vehicle velocity and the level of the vibration given to the tire from the road surface have no influence on the resonant frequency of a tire in the torsional direction but have a very high influence on the entire frequency characteristic.
On the other hand, when a wheel rotation signal is used as a time-series signal and a resonant frequency is estimated based on a linear prediction model, it is very difficult to estimate a true resonant frequency without being influenced by frequency components other than the resonant frequency. Even when there is only one resonance point depending on the air pressure, it is very difficult to accurately estimate the resonance point by a low-order model such as the second-order model as in the method disclosed in the specification of Japanese Patent No. 3152151. Even when a time-series signal is subjected to a filtering process having a predetermined passband, it is still difficult to estimate the resonance point without being influenced by a change in the frequency characteristic.
In the case of the method disclosed in Japanese Unexamined Patent Publication No. 2005-14664, if resonant frequencies estimated under the completely same conditions and at a normal internal pressure are stored, a decrease in a tire air pressure can be accurately detected. However, it is difficult to run a vehicle at a normal internal pressure under all combinations of conditions. Consequently, when there is no resonant frequency (reference value) to be compared with, there is no choice but to make a comparison with a resonant frequency obtained by interpolation for example, thus high accuracy can not be expected.